The impact of receiver's tilted angle on the channel capacity in visible light communications (VLCs) is investigated. First, the system model is analysed. On the basis of the system model, the lower and upper bounds on the channel capacity for the VLC are presented. An optimisation problem is formulated to improve the channel capacity by tilting the receiver plane. The proposed problem is shown to be a convex optimisation problem, which is efficiently solved by using the CVX toolbox for MATLAB. Numerical results show that the channel capacity can be dramatically improved by tilting the receiver plane properly.

Recently, the spatial modulation technique has been proposed for visible light communication (VLC). This paper investigates the average symbol error rate (SER) for the VLC using an adaptive spatial modulation (ASM). In the system, the analysis of the average SER is divided into two aspects: the error probability of the spatial domain and the error probability of the signal domain when the spatial domain is correctly estimated. Based on the two aspects, the theoretical expression of the average SER is derived. To further improve the performance of the system, an optimization problem is proposed to optimize the modulation orders on the LEDs. The ASM-based and the candidate-reduction-ASM-based optimization algorithms are proposed to solve the problem, respectively. Numerical results show that the derived theoretical values of the average SER are quite accurate to evaluate the system performance. Moreover, compared with the existing schemes, the proposed two algorithms are better choices for the VLC.

As a power and bandwidth efficient modulation scheme, the optical spatial modulation (SM) technique has recently drawn increased attention in the field of visible light communications (VLC). To guarantee the number of bits mapped by the transmitter's index at each timeslot is an integer, the number of transmitters (i.e., light-emitting diodes) in the SM based VLC system is often set be a power of two. To break the limitation on the required number of transmitters and provide more design flexibility, this paper investigates the SM based VLC with an arbitrary number of transmitters. Initially, a channel adaptive bit mapping (CABM) scheme is proposed, which includes three steps: bit mapping in space domain, bit mapping in signal domain, and the channel adaptive mapping. The proposed CABM scheme allows operation with an arbitrary number of transmitters, and is verified to be an efficient scheme through numerical results. Based on the CABM scheme, the information-theoretical aspects of the SM based VLC are analyzed. The theoretical expression of the mutual information is first analyzed. However, it is very hard to evaluate system performance. To obtain more insights, a lower bound of the mutual information is derived, which is in closedform. Both theoretical analysis and numerical results show that the gap between the mutual information and its lower bound is small. Finally, to further improve the system performance, the precoding scheme is proposed for the SM based VLC. Numerical results show that the system performance improves dramatically when using the proposed precoding scheme.

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